Crystalline polypropylene is excellent in mechanical property, chemical resistance, etc. and therefore, is widely used in various molding fields. However, a homopolymer of propylene or a random copolymer with a small amount of α-olefin sometimes lacks impact resistance, though its rigidity is high.
To cope with this problem, attempts have been made to improve the impact resistance by a method of adding a rubber component such as ethylene-propylene copolymer (EPR) to a propylene homopolymer or by the production of a so-called impact copolymer where a rubber component is incorporated by copolymerizing propylene and ethylene or α-olefin continuously after the homopolymerization of propylene. Furthermore, flexibility and impact resistance can be enhanced by increasing the amount of a rubber component in the impact copolymer.
As another problem, in an impact copolymer obtained by the polymerization in the presence of a conventional Ziegler-Natta catalyst, a low-molecular-weight component (an oligomer component, etc.) is present by the nature of the catalyst. Particularly, in recent years, there is a tendency to more improve the moldability of the obtained impact copolymer by increasing the flowability.
However, when the flowability of the rubber moiety is raised too much, this is accompanied also by an increase in the production rate of a low-molecular-weight component, and the low-molecular-weight component is known not only to give rise to generation of fume, bad smell, etc. during processing but also to, even after processing, adversely affect the odor or taste and cause various problems such as change for the worse in the blocking property due to sticking. Deterioration of powder properties of the polymerized polymer disadvantageously makes stable production impossible. On the other hand, when the difference in the average molecular weight between crystalline polypropylene and rubber moiety becomes large, a problem such as increased incidence of gels in a molded article or high coefficient of linear expansion of a molded article is caused.
Meanwhile, it is known that isotactic polypropylene is obtained by polymerizing propylene with use of a metallocene-based catalyst different from the conventional Ziegler-Natta catalyst. It is also known that an impact copolymer is produced by copolymerizing ethylene and propylene with use of a similar catalyst continuously after the homopolymerization of propylene (see, for example, Patent Documents 1 and 2). An impact copolymer having good rigidity and good impact resistance is also disclosed (see, for example, Patent Document 3).
Among others, the impact copolymer needs to exhibit, for example, a lower glass transition temperature so as to develop high impact resistance and for satisfying this requirement, it is supposed that copolymerization of propylene and ethylene or α-olefin is preferably performed to let each content satisfy a certain range (see, for example, Non-Patent Document 1).
As for the transition metal compound constituting the above-described metallocene-base catalyst, many examples have been already known. Above all, in order to improve the rigidity of an impact copolymer, a transition metal compound capable of providing a homopolypropylene having a high melting point has also been already known (see, for example, Patent Document 4).
However, the production of such a propylene-based impact copolymer by the use of a metallocene-based catalyst involves the following technical problems due to difference in the reactivity of propylene with other comonomers.
When copolymerization of propylene and ethylene or α-olefin is performed after the homopolymerization of propylene according to the conventional production method by using a metallocene-based catalyst, the gas composition ratio of propylene/(ethylene or α-olefin) in the polymerization atmosphere greatly differs from the polymerization amount ratio of propylene amount/(ethylene or α-olefin amount) polymerized in this atmosphere, and the polymerization amount of (ethylene or α-olefin) in the polymer sometimes decreases. That is, in order to obtain a copolymer having a desired ethylene or α-olefin content, polymerization must be performed by feeding a gas having a monomer ratio greatly different from the contents in a copolymer, which involves a production problem. In an extreme case, a copolymer having desired contents cannot be produced due to restriction of the polymerization apparatus.
In this way, a catalyst using a metallocene complex produces a large difference in the ethylene content between an ethylene/propylene mixed gas and a polymer, and it is demanded to develop a production method where the difference is eliminated and the uptake efficiency of ethylene and α-olefin is high.
In addition, in the case of using a so-far known metallocene catalyst, there is a problem that when copolymerization of propylene and ethylene or α-olefin is performed in gas phase, the molecular weight of the obtained copolymer is low. In a propylene-base impact copolymer, the molecular weight of the copolymer must have a value not less than a certain level so as to develop high impact resistance, and a production method capable of producing a copolymer having a high molecular weight is demanded as well. Furthermore, development of a catalyst with high rubber activity is also demanded for the purpose of reducing the catalyst cost per unit polymer or increasing the content of rubber moiety.
As described above, in order to enhance the rigidity of an impact copolymer, a homopolypropylene having a high melting point is required. However, as far as a catalyst using a metallocene complex is concerned, in the above-described catalyst capable of enhancing the uptake efficiency of ethylene and α-olefin and producing a copolymer having a high molecular weight, a catalyst exerting a sufficient performance as a catalyst capable of producing a homopolypropylene having a high melting point is not yet known.
Patent Document 5 discloses a metallocene complex having a substituent on the 5-position of an indenyl ring and having, on the 2-position of an indenyl ring, a furyl or thienyl group that may have a substituent, and a metallocene complex with which a relatively high ethylene uptake efficiency and a copolymer having a high molecular weight can be provided, is disclosed.
Patent Document 6 discloses a metallocene complex having a cyclic-structure substituent between the 5-position and the 6-position of an indenyl ring and having, on the 2-position of an indenyl group, a furyl or thienyl group that may have a substituent, and a metallocene complex with which a relatively high ethylene uptake efficiency and a copolymer having a high molecular weight can be provided, is disclosed.
However, the metallocene complexes disclosed in Patent Documents 5 and 6 do not have a sufficiently high performance in terms of activity, and creation of a higher-performance metallocene complex is demanded. The melting point is also preferably higher, because the rigidity can be enhanced.